THE MEASUREMENT OF HIGH TEMPERATURES 233
back to pyrometers of this description is the uncertainty of always obtaining the same degree of intensity from the standard light, althougli the latest Wanner instrument is provided with. electric lamp, galvanometer, and regulating resistance by means of which it is claimed that intensity can always be maintained. at the prescribed standard.
THE CAMBRIDGE OPTICAL PYROMETER
This pyrometer (Fig. 132) is a convenient and accurate instrument which
utilizes the relatiouship between the intensity of the red rays emitted by heated bodies
and their temperatures. Into the enlarged part in the front of the pyrometer is fitted an electric lamp, and a screen at the back end of this portion contains two holes.
Light from the obj eet under observation passes through one hole, and light from the lamp through the other. These beams of light then pass through, a system of lenses and prisms, are polarized in ■different planes, and are rendered monochromatic. Finally, the two beams of liglit pass through to single eye-piece, and the observer sees an illuminated circular field divided into two semicircles. One
semicircle is filled by all image Fig. 132.—The Cambridge Okhcal Pyrometer.
of the hot body under observa-
tion, and the other is uniformly illuminated by the electric lamp. The two semicircles are brought to an. equal intensity of illumination by turning the eye-piece, to which the scale pointer is direetly attached. In this manner the un.kn.own rays are compared with those of known intensity from the electric lamp. Means are introduced to ensure that, whatever the voltage of the lamp battery may be, the current passing through the lamp is constant. Tlius constant candle-power of the standard is maintained.
RESISTANCE PYROMETERS
The electrical resistance of a metallic wire varies with its temperature ; conse-■qucntly, this property presents a method of measuring temperature. Sir W. Siemens was the first to endeavour to construct a platinum resistance pyrometer. It consisted ■of a platinum wire wound round a fireclay cylinder, the whole being protected by a stout wrought-iron tube. The instrument, however, was not altogether satisfactory, and the question. was ultimately taken up by Professor Callendar, who pointed out that if a coil of pure annealed platinum wire be wound on a mica framework, and thoroughly protected from the action of gases, then the temperature as deduced from an observation of the resistance of the wire will seldom be in error by more than